Is there a 'prime' CC or HP?

50cc is plenty to assist and they're under 2 horsepower. I go 25mph sustained on my 50cc when i have it geared down to 36 or 34 t but i have to pedal up to the powerband

49cc is just under the legal cc limit for a lot of areas, plus the engines (at least the chinese two-strokes) seem a bit better balanced overall.

33 cc friction drive is probably the least. thats less than 1 horsepower. the most would probably be around 10hp. thats the bigger morinis, the arrow motors, the 212 cc that scotto built is 7 stock but a whole lot more the way it was retooled.
what are you talking about prime? and sweet spot? that is not good terminology for the way you're analyzing this.
Just build what you're going to build,instead of taking what other people build and find a middle ground. Just build to your capability and your preference. I think you'll find that if you get a good gear ratio that you can pull yourself and a suitcase full of bricks with the smallest of motors.

a lot has to do with th e transmission, and if you have selectable gears, 1 hp is enough with selectable gears for 0-30

Not very many typical bicycles are going to last very long with more than 2 hp.

Trey said:

I didn't even think to make a distinction between 2 or 4 stroke. I may be looking at this wrong? I've got a 5HP Tecumseh, for example. After researching, and talking to a bunch of people, my guess is that it's about 150CC. I wouldn't even consider a 150CC kit, that's more Scotto's deal, but I think a 5HP should be just more than I need, which means I could pack a little more weight. My project will need to do that. This whole thing got me thinking about CC vs HP, and what the numbers mean, practically. With the example given originally, I would guess (laws not withstanding) that there's a general minimum that would propel that bike reasonably. That makes me wonder if there is a 'prime zone' for CC or HP...
Thanks for the replies, I'm trying to understand things better.

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In all of my findings so far.........I personally think a 79-99cc 4-stroke is the overall best size for a motorized bicycle for everyday use on the street. A healthy 49cc 4-stroke is fine if you don't have hills to tackle........on flat land without multiple gears, this would be fine.

And I would never put anything larger than a 212cc 4-stroke on a bicycle for any reason other than maybe for a tandem.........period!

This is my 4-stroke perspective only.....PM me for 2-stroke details........



EDIT: this equates to 3-4hp being pretty much ideal in my mind (unless it's a "RACE" bike).

Trey said:

Thanks! I wanted to have some idea on when to reign in my imagination, before I waste too much planning time.
Saw a pic of your knee injury not long ago somewhere here, hope your up and running again man.

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What this?



It was just a couple of deep surface scratches from 3/4" gravel and whatnot. That pic was taken a couple days ago, it looks much better now that it's been two weeks

Go 4-stroke and don't go broke.........I'm ready to race!

Well crud muffins... that's a hard act to follow.
Sry to see u picked up some road rash, scotto.

IMO, 100cc and 4hp would provide a decent, reliable, low powered, personal transport Motor Assisted Bicycle for use in most cities and towns that only have moderate traffic situations, distances, and slight roadway gradients.

Of course it can be done with less.
IMO, 100cc and 4hp would be 'primo', before taking the step into the domain of motorcycles with gears and all that stuff.

Best
rc

...Oh, and I'm figuring on a 300lb total weight... bike, rider, cargo.

the china girls can be modified up to 9.5 hp, as proven by arrow cycles, however then reliability becomes a concern. My HT has been running for a year so far, and while nothing major has occurred with it, I ride every day, for several hours, sometimes nonstop, WOT, doing 50-60kph. Exhaust and High comp head make the biggest difference IMHO. In terms of power vs size, depends on the quality and tuning of the motor, but even then, apparently a 9.5hp morini geared at 36T can do 80mph+ all day long, but will still need assistance at lower speeds. People here are right, it's mostly the gearing that will determine what you want. 4-strokes tend to make more low end power, so they'd be better for stop-n-go's, as well as apparently more reliability with them. 2-strokes tend to produce more power per CC, but premixing your gas tends to be annoying as well as tuning variances can drastically affect the power output. a water cooled 50cc morini punches out 11.5hp, but all that power tends to be at the high end, so launches suffer. Depending how you tune the HT's, you could tune for low end, midrange, or high end power, the tendency is the higher your powerband, the more HP you get (it's torque x RPM/5252 if I remember right), however having 9.5 hp at the high end could easily leave you with 0.2hp at launch/idle speeds. a high gearing will let you get to that peak faster and give you more torque to the wheels, but your top speed will suffer. I could lanch without assistance on my 36T on my HT but it requires a lot of feathering, and honestly its not that great for the clutch.

So, it depends what you're going for really, but the biggest differences in terms of raw launching power would be in your gearing, not the motor.

So I suppose in a nutshell, the power you get is directly affected by compression. That is the more you can pack into the cylinder to be squashed, the more power you will get, so the more CC, the more you can cram in. Exhaust, Reeds and High comp head will all affect these greatly, 4 strokes do it by valve timing though.

Trey said:

Scotto- one more quick question for you. If you could only find motors rated in HP, what would you choose to build the equivalent of the bike you describe here? Thanks

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He would say about 55 hp.

I will chime in: agree with scotto four stroke 79 or 99cc. You can also hope them up with a cam and head work. Rode mine today all over with no issues. It's nice to pull up to the gas station and fill ER UP...

Trey said:

Scotto- one more quick question for you. If you could only find motors rated in HP, what would you choose to build the equivalent of the bike you describe here? Thanks

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I think 3-5hp is ample for any street cruiser, 4 or 2-stroke. Oh...and 4-strokes rule!

55hp would suit me fine on a Honda CRF450R..........not on a bicycle......honestly.